The objectives of this project are to define the role of renal interstitial hydrostatic and colloid osmotic pressure (COP) in regulation of tubular sodium reabsorption. All previous studies of Starling forces across renal capillaries fail to take interstitial COP into consideration, therefore, net capillary COP has never been estimated. Renal lymph flow and protein concentration are considered representative of interstitial volume and COP. In one series of saline loaded dogs interstitial protein concentration will be the only variable for the first portion of the experiment. In later stages hyperoncotic solutions will be infused, thereby changing both capillary and interstitial COP. Changes in net capillary COP will be correlated with fractional sodium reabsorption during saline loading. In the second series both net capillary hydrostatic and net capillary COP will be increased. These experiments will utilize both vasodilator drugs and hyperoncotic infusions during water diuresis and should help delineate the sites within the tubule where physical factors affect sodium reabsorption.